1. Field of the Invention
The present invention relates generally to the fields of synthesis and polymerization of xcex1,xcex1xe2x80x2-dihalo-p-xylenes. More particularly, it concerns the use of a novel synthesis technique and the use of a novel anionic initiator that allows control of the molecular weight of the polymer.
2. Description of Related Art
Poly(p-phenylene vinylene)s (PPV) are of considerable interest for applications in light-emitting diodes (Friend and Greenham, 1998), field effect transistors (Parker et al., 1993), and photovoltaic devices (Antoniadis et al., 1994). Two approaches have generally been used in the art for the synthesis of PPVs: the Wessling route (Wessling and Zimmerman, 1968; 1970) and the Gilch route (Gilch and Wheelwright, 1966). The Wessling route involves treatment of p-xylylene sulfonium salts with an equal molar amount of base to form a soluble precursor polymer. The precursor polymer is then thermally treated to give the conjugated PPV. The Gilch route employs the treatment of xcex1,xcex1-dihalo-p-xylenes with potassium tert-butoxide in organic solvents. Alkyl or alkoxy substituents on the aromatic ring are often used to impart solubility to the PPV. One of the most widely studied PPVs is poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) due to the enhanced solubility of this polymer (Wudl and Srdanov, 1993).
The mechanism of polymerization of MEH-PPV is not fully understood and several processes have been suggested (Moratti, 1998). The polymerization is believed to proceed through a reactive quinodimethane intermediate that has been observed from UV spectra (Lahti et al., 1988; Denton et al., 1992). However, the nature of the propagating species, radical or anionic, is not firmly established. Several reports have suggested a radical polymerization due to decreases in molecular weight with the addition of chain transfer agents such as anthracene or TEMPO (Denton et al., 1992; Issaris et al., 1997; Hontis et al., 1999). In contrast, the addition of 4-tert-butylbenzyl chloride resulted in an inverse relationship between the amount of benzyl chloride and molecular weight suggesting an anionic polymerization (Hsieh et al., 1997; Hsieh et al., 1998). Ambiguities in the polymerization mechanism reflect a need for the development of novel and better methods for the polymerization of xcex1,xcex1xe2x80x2-dihalo-p-xylenes.
The present invention overcomes these and other deficiencies in the art. Novel polymerization methods for the polymerization of xcex1,xcex1-dihalo-p-xylenes are described herein that provide methods to control the molecular weight of the polymer and to circumvent polymerization problems such as gelation or precipitation of polymer products during polymerization.
The inventors also describe studies that elucidate the polymerization mechanism of xcex1,xcex1-dihalo-p-xylenes. In one embodiment, the mechanism of polymerization of xcex1,xcex1xe2x80x2-dihalo-2-methoxy-5-(2-ethylhexyloxy)-xylene""s is elucidated. In specific embodiments, the mechanism of polymerization of xcex1,xcex1xe2x80x2-dichloroxe2x80x2-2-methoxy-5-(2-ethylhexyloxy)-xylene; xcex1,xcex1xe2x80x2-dibromo-2-methoxy-5-(2-ethylhexyloxy)-xylene; xcex1,xcex1xe2x80x2-di-iodo-2-methoxy-5-(2-ethylhexyloxy)-xylene; and xcex1,xcex1xe2x80x2-difluoro-2-methoxy-5-(2-ethylhexyloxy)-xylene are described.
The inventors have further described agents that initiate and control the polymerization reaction. These agents affect the polymerization mechanism of the reaction and thereby control the molecular weight of the resulting polymer. In one aspect, the agent is an anionic initiator. In a specific aspect, the anionic initiator is a nucleophilic anion derived from alkyl alcohols, phenols, mercaptans, and thiophenols. 4-methoxyphenoxide is one such initiator which is preferred for the methods of this invention.
In embodiments concerning the polymerization of xcex1,xcex1-dihalo-p-xylenes, the order of addition of reagents used for the synthesis i.e. base to monomer or monomer to base are defined. Bases include but are not limited to metal hydrides such as sodium hydride or calcium hydride, alkyl metals such as butyl lithium, amides such as sodium amide or lithium diisopropyl amide, and alkoxides such as potassium tert-butoxide or sodium iso-propoxide with potassium tert-butoxide as the preferred base. In a specific embodiment, the order of addition of reagents for the polymerization of xcex1,xcex1xe2x80x2-dibromo-2-methoxy-5-(2-ethylhexyloxy)-xylene is described.
Other embodiments describe methods used to follow the progress of polymerization. For example, the use of a rheostat as a mechanical stirrer to detect in-situ changes in viscosity and to follow the polymerization process are described.
Thus, one embodiment describes methods for the polymerization of xcex1,xcex1xe2x80x2-dihalo-p-xylene""s comprising: a) obtaining monomers of xcex1,xcex1xe2x80x2-dihalo-p-xylene""s; b) adding a base to the monomers; c) adding an anionic initiator; and carrying out the polymerization. In one aspect of the invention the method yields polymer at about 66%.
In one embodiment of the invention the temperatures used for the polymerization had a range of from about xe2x88x9240xc2x0 C. to 100xc2x0 C. Thus, the reaction may be carried out at xe2x88x9240xc2x0 C. xe2x88x9235xc2x0 C., xe2x88x9230xc2x0 C., xe2x88x9225xc2x0 C., xe2x88x9220xc2x0 C., xe2x88x9215xc2x0 C., xe2x88x9210xc2x0 C., xe2x88x925xc2x0 C., 0xc2x0 C., 5xc2x0 C., 10xc2x0 C., 15xc2x0 C., 20xc2x0 C., 25xc2x0 C., 30xc2x0 C., 35xc2x0 C., 40xc2x0 C., 45xc2x0 C., 50xc2x0 C., 55xc2x0 C., 60xc2x0 C., 65xc2x0 C., 70xc2x0 C., 75xc2x0 C., 80xc2x0 C., 85xc2x0 C., 90xc2x0 C., 95xc2x0 C., 100xc2x0 C. The preferred temperature for the polymerization is 25xc2x0 C.
In another embodiment, the xcex1,xcex1xe2x80x2-dihalo-p-xylene is xcex1,xcex1xe2x80x2-dihalo-2-methoxy-5-(2-ethylhexyloxy)-xylene. In specific embodiments, the xcex1,xcex1xe2x80x2-dihalo-2-methoxy-5-(2-ethylhexyloxy)-xylene is xcex1,xcex1xe2x80x2-dibromo-2-methoxy-5-(2-ethylhexyloxy)-xylene, xcex1,xcex1xe2x80x2-dichloro-2-methoxy-5-(2-ethylhexyloxy)-xylene, xcex1,xcex1xe2x80x2-di-iodo-2-methoxy-5-(2-ethylhexyloxy)-xylene or xcex1,xcex1xe2x80x2-di-fluoro-2-methoxy-5-(2-ethylhexyloxy)-xylene.
In one aspect of the method, the base is non-nucleophilic. Thus, the base can be an alkoxide. In a preferred aspect the alkoxide is potassium-tert-butoxide. In an alternative aspect the alkoxide is sodium iso-propoxide.
In alternative embodiments, the base is a metal hydride. In more specific aspects of this embodiment the metal hydride is sodium hydride or calcium hydride.
In still other alternative embodiments, the base is an alkyl metal. In specific aspects of this embodiment, the alkyl metal is butyl lithium.
In yet other alternative embodiments, the base is an amide and the amide is sodium amide or lithium diisopropyl amide.
In one embodiment the anionic initiator is a nucleophile. The anionic initiator can be any nucleophilic anion derived from alkyl alcohols, phenols, mercaptans, and thiophenols. In a preferred embodiment 4-methoxyphenoxide is the initiator of choice.
In another embodiment a method for the polymerization of xcex1,xcex1xe2x80x2-dihalo-2-methoxy-5-(2-ethylhexyloxy)-xylene""s is described wherein the molecular weight of the polymer can be controlled. This is accomplished by the use of an anionic initiator compound. The initiator compound is a neutral nucleophile such as an amine or a nucleophilic anion derived from alkyl alcohols, phenols such as 4-methoxyphenoxide, mercaptans, thiophenols or amines. The method also allows processability control. In other aspects the method includes monitoring the progress of the polymerization process by the use of a rheostat as a mechanical stirrer to detect in-situ changes in viscosity of the polymer formed.
As used in the specification and claims the words xe2x80x9caxe2x80x9d and xe2x80x9canxe2x80x9d when used in combination with the conjunction xe2x80x9ccomprisingxe2x80x9d denote xe2x80x9cone or more than onexe2x80x9d.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.